Adjustable dosage syringes have been used in the art for either adjusting the dosage for a particular administration of the contents of the syringe and/or for delivering multiple dosages of the contents of the syringe in multiple separate injections. For example, in veterinary use, the dosage of a particular drug administered to an animal may vary considerably with the weight of an animal. Thus, when a herd of livestock is to be injected with a drug, the careful and sterile loading of a syringe in the field is not always practical, and it is conventional in the art to provide the veterinarian with a syringe capable of adjustment of the dosage delivered from that syringe in injecting individual animals. These syringes are normally disposable syringes and, accordingly, the veterinarian can select the proper dose for an individual animal, rapidly set the syringe for delivering only that dose, inject the animal, and dispose of the syringe with the remaining drug content therein. Alternatively, when a drug is to be administered to a large number of animals, multiple doses for a number of animals may be contained in a single syringe and the veterinarian can select the proper dosage for each animal, rapidly set that dosage on the syringe and serially inject the animals with the proper dosage.
Since a large number of animals are often treated in a short period of time, it is most important that the adjustable syringe be capable of rapid adjustment for dosage delivered and be accurate in the dosage. Further, since these syringes are normally disposable, it is imperative that the syringes be of such construction that they are inexpensive to manufacture. Further, since it is conventional in the art that the syringe and drug be supplied as a total package from the manufacturer, the adjustable syringe must be capable of delivering dosages of varying amounts, consistent with the weight of the animal, and that the increment of dosage with which the syringe may be set is relatively small.
A number of syringes of this nature have been made available to the art. Thus, U.S. Pat. No. 3,563,240 provides a syringe with a plunger having peripheral thread thereon, and a cooperating threaded nut. By threading the nut up and down of the syringe plunger, adjustable dosage of the drug contained in the syringe may be delivering by way of the nut limiting the depression of the plunger into the syringe barrel. However, manually threading the nut up and down the plunger is relatively time-consuming, particularly when the dosage requirement from animal to animal varies considerably and the threading of the nut must be accurately performed in order to ensure that the correct dosage is given. Manufacture of such a syringe is also expensive, since it requires manual manipulation to thread the nut on the plunger during the assembling process.
Efforts have been made in the art to overcome such disadvantages, notable among which is the known "split-ring" syringe wherein the "nut" is releaseably hinged about the circumference thereof. Whereby, the nut can be opened, manually slid to the portion of the plunger desired for the appropriate dose, closed on the threads, and the appropriate dose discharged from the syringe. However, here again, this requires manual opening and closing of the hinged "nut" and can be time-consuming for the reasons expressed above.
Older approaches in the art avoided some of the problems discussed above by providing a plunger with various types of replaceable stop means, e.g. pins, clips and the like, but these approaches were too cumbersome for field use. Another approach in the older art was that of providing indentations in a ring disposed about the plunger whereby the indentations would allow the ring to be lifted and moved along the plunger to discrete recesses in the legs of the plunger wherein the ring could be dropped into and set the dosage discharged by the plunger. U.S. Pat. No. 2,856,925 is representative thereof. This older approach in the art, while being relatively rapid to operate, suffered from the disadvantages that the dosage was fixed by the size of the recesses in the legs of the plunger, and the necessary clearance in the recesses for allowing the ring to drop thereinto could not accurately set the dosage discharged from the syringe.
Accordingly, while the advances in the art have improved the accuracy of the dosage which can be delivered, i.e. by virtue of the threaded nut arrangement and the like, these advances have been accompanied by less convenient and rapid use of the syringes. It would therefore be of considerable advantage in the art to provide an adjustable dose syringe which can not only be rapidly changed to set the dosage delivered but can very accurately set that dosage and provide for very small increments of dosage adjustment.